I’ve already touched on this topic on the sister SE site once or twice. To summarize what is told there (check for more detail), there would be several considerations as related to energy use and sustainability:
What one can do (and what I do) is to turn off engine during long traffic lights, railway crossings and so forth. Anything more than 10 to 15 seconds (very generously) would save fuel, because car that is already warm does not need that much effort to re-start, as it is at the proper operating temperature, and oil is distributed around the engine and at a high lubricity. Motor stays within the efficient range of temperatures for more than few minutes, and takes several hours to completely cool down (depending on the weather), so there is definitely no need to keep the car running during short errand (never mind someone could simply run off with the vehicle).
Any concerns about premature wear of the starting infrastructure are moot considering savings on fuel and other engine components (one could afford to replace starters every 3-4 years at the worst). See the linked posts for detailed arithmetic. And any energy losses that occur during starting, and keeping the electrical loads going during the time the engine is off are minuscule compared to energy used by petrol combustion while idling. See relevant question at Physics.SE.
I tried this for three months (turning off the engine at traffic lights). The result is that my car battery stopped working and there was smoke coming from under the bonnet. I then had to arrange to have the car towed to a garage. I was told that I would need a new starter motor at a cost of £150. It certainly doesn't represent a saving.
I tried this method on my old 323 manual early last year for about 3 months with no issues but then that car went in for a normal service and I was forced to borrow my mother's smaller Jazz auto for a few days. It broke down the 2nd day of this use, the battery had gone dead. This car had a good history and the battery wasn't that old. In fact the battery was able to be recharged and is still in use today.
My uncle who's a mechanic said this method was bad for the car (no specifics). I also stopped using this method with my old 323 manual as I wanted to sell it soon in good working order (although it did gain an unknown motor issue about this time that fast forwarded my selling it, a mechanic bought it). I won't using this method with my new auto. I can't risk breaking down on my way to work again and I don't want to damage my nice new i30. I would like to be convinced otherwise if you have good evidence, I want to save fuel and reduce my carbon footprint.
I'm trying this out just now, car in question 1.2 carburetor astra mk 4/ astra g estate/caravan/station. Thus far I seem to be making a saving, (as long as I allow the engine to warm up a little) but it's entirely possible my general fuel conscious driving is making more of a difference than turning off the engine.
*related though another matter... Coasting down hills is probably saving more fuel than shutting off at traffic lights, and though not very safe certainly avoids premature starter wear by virtue of leaving the ignition on and bump starting when I reach the bottom of the hill.
No, you won't save any fuel. You should remember that it takes a lot of energy to start the engine. This intially comes from the battery. However as soon as the engine starts this energy must be replenished by the engine and this is more than is required to crank the engine (because the battery is only 70% efficient).
Also all electrical loads are supplied by the battery when the engine is stopped. When the engine is running these loads are from the alternator (less losses). When you start your engine, you have to drive something like 5-8 miles to recharge. So if you are stopped at traffic lights for 5 minutes you have to add this to the 5 mile equivalent. So in reality it can cost more fuel! People are wrong thinking they can get something for nothing, like the high MPG of hybrid cars where the diesel or petrol engine is driving an alternator, loss 70%. rectifier, controller and motor loss 30%, instead of the engine driving the wheels directly.
I have been a great fan with this technology, however after some thought, I find that there are some drawbacks with this tech.
The part that made this work is a special starter that starts faster. So I assume it draws more power than normsl starter. So, I think this will eventually kill a battery faster unless thr battery is more powerful, which in turn becomes more expensive... meh...
Then there's the problem when starter reliability fails. It would cost more to replace this powerful starter.. meh..
Then there is doubt on the actual savings once you take in the recharging cost of fuel to power this start stop. Conventional cars only charge after the start of a car once in it's journey.. how much fuel used? I'm not sure.. but if you keep restarting the engine surly it will need to recharge more.. so, probably the savings wont be as much...
I have been using this method for years, on both my Toyota and my Ford Explorer. I save lots of gas, nothing has ever happened to my batteries, and have never had a mechanical problem. You need to use common sense. I typically shut it off when it's a long light and I'm in a long line, railroad crossings, and any other situation where it's going to be at least a minute wait. People seem to panic when shutting off their car in traffic, thinking some mechanical issue is going to happen, the battery will all of a sudden go bad, etc., what's the difference when shutting the car off at the mall? Then going to the grocery store and shutting it off there? The responders in this article make me laugh when they say they tried it and all of a sudden the battery went bad and there were mechanical problems. I'm pretty sure if the battery went bad in a parking lot somewhere or if a mechanical problem developed, that it was just normal wear and tear. Listen up folks, starting a car in the roadway is not different than starting it in your driveway or the parking lot of a grocery store. Use your head.
Nobody who turns off their engine ever mentions the fact that restarting their engine causes a slight delay in getting away and must result in increased congestion. This will increase the fuel consumption of everyone in the queue who should have got away but didn't and those who had to wait a few moments longer.